Closes #15621: User notifications (#16800)

* Initial work on #15621

* Signal receiver should ignore models which don't support notifications

* Flesh out NotificationGroup functionality

* Add NotificationGroup filters for users & groups

* Separate read & dimiss actions

* Enable one-click dismissals from notifications list

* Include total notification count in dropdown

* Drop 'kind' field from Notification model

* Register event types in the registry; add colors & icons

* Enable event rules to target notification groups

* Define dynamic choices for Notification.event_name

* Move event registration to core

* Add more job events

* Misc cleanup

* Misc cleanup

* Correct absolute URLs for notifications & subscriptions

* Optimize subscriber notifications

* Use core event types when queuing events

* Standardize queued event attribute to event_type; change content_type to object_type

* Rename Notification.event_name to event_type

* Restore NotificationGroupBulkEditView

* Add API tests

* Add view & filterset tests

* Add model documentation

* Fix tests

* Update notification bell when notifications have been cleared

* Ensure subscribe button appears only on relevant models

* Notifications/subscriptions cannot be ordered by object

* Misc cleanup

* Add event icon & type to notifications table

* Adjust icon sizing

* Mute color of read notifications

* Misc cleanup

netbox/project-static/styles/custom/_notifications.scss

@@ -0,0 +1,9 @@
+@use 'sass:map';
+
+// Mute read notifications
+tr[data-read=True] {
+  td {
+    background-color: var(--#{$prefix}bg-surface-secondary);
+    color: $text-muted;
+  }
+}

netbox/project-static/styles/netbox.scss

@@ -24,3 +24,4 @@
 @import 'custom/interfaces';
 @import 'custom/markdown';
 @import 'custom/misc';
+@import 'custom/notifications';